Continuous electrodes



Jan. 23, 1968 e .1. R. ALEXANDER 3,365,533

CONTINUOUS ELECTRODES Filed Feb. 23, 1967 2 SheetsSheet 1 P i G. 3

INVENTOR JOHN R ALEXANDER AGENT Jan. 23 1968 J. R. ALEXANDER" 3,365,533

CONTINUOUS ELECTRODES 2 Sheets-Sheet 2 Filed Feb. 23,- 1967 FIG. 5

INVENTOR JOHN R. ALEXANDER BY 4% AGENT United States 3,365,533 CGNTENUOUS ELECTRODE;

John R. Alexander, St. Louis, Mo., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware Filed Feb. 23, 1%7, Ser. No. 618,175 Claims. (Cl. 13-18) ABSTRACT 6F THE DISCLOSURE This invention relates to improved novel electrodes for electric furnaces and to processes utilizing them. More specifically, it relates to an improved novel electrode for electric furnaces utilizing carbon electrodes in the processing and treatment of raw materials which are electrically reduced.

There are a larw number of processes which use electric furnaces such as processes for the production of elemental phosphorus, iron, and aluminum from ores containing their respective oxides. Other well known processes are those in which alloys of various metals are produced such as alloys of silicon, chromium, manganese, cobalt and the like and for the production of certain chemicals such as calcium carbide, alumina and thelike.

The electrodes most commonly used in electric furnaces are carbon electrodes. These electrodes are continuously consumed during normal operations because of the extremely high heat and the corrosive gas produced from raw materials. The pre-baked solid carbon electrode is one of the types that has generally been used heretofore and is fabricated as a unit prior to insertion into the furnace. As the electrode is consumed, continuous columns are built by using various connecting means to join the individual pieces together. In some instances, particularly when the electrode is relatively large in diameter, that is excess of about 4 feet, the connecting means is often inadequate and often results in a portion of the partially consumed electrode dropping into the furnace. The electrode breakage results in high electrode consumption and unsatisfactory furnace operation.

An additional type of electrode has been used in some instances and is referred to herein as the self-baking type of electrode. This electrode consists of an electrically conductive casing and fins extending inwardly for supporting a paste which is electrically and thermally conductive but which is thermally sensitive and partially decomposes to form carbon. The paste consists of relatively small carbon granules and hydrocarbons. This mixture is relatively plastic at the temperatures at which it is placed into the electrode which generally are in the range of from about 0 to about 200 C. The higher temperatures which exist inside the furnace cause the hydrocarbons to vaporize and to decompose to carbon. The vaporized material escapes downwardly, the hydrocarbons are decomposed to carbon due to the high temperature and the carbon deposits in the voids between the carbon granules. A carbon tip is thereby produced which is extremely resistant to the high temperatures and corrosive gases inside the furnace. The gases inside the furnace are quite corrosive upon most materials from which the casings are made, particularly at the high tematct peratures that exist in the furnaces, therefore, holes tend to be corroded in the casing. The vaporized hydrocarbons and the hydrogen which evolves during decomposition of the hydrocarbons escapes through the holes in the casing and not downwardly through the tip of the electrode. The escape of either the vaporized hydrocarbons or the hydrogen in this manner results in a carbon which is less resistive to the hot gases and high temperatures. The electrodes are consumed more readily and in some instances the lower portion breaks resulting in unsatisfactory furnace operation. The self-baking electrodes also may be constructed in a continuous manner by building additional columns of the casing and fins and then filling the columns with the electrode paste.

The problems with electrode breakage and electrode consumption become more acute with both types of electrodes as the diameter of the electrode increases because current density at the tip of the electrode increases as the diameter increases. It has been reported that electrode burn-off increases approximately as the ratio of the square of the current density after an optimum current density is reached. The optimum current density will depend upon the particular furnace design and the particular process employed. Electrode breakage, that is, the dropping off of major portions of the electrode also increases as the diameter of the electrode increases because of the higher temperatures created by the higher current densities.

The present invention overcomes many of the difficulties heretofore encountered by the previous electrodes by insuring that the carbon which is formed upon the decomposition of the hydrocarbons is deposited in the voids between the carbon granules in the paste.

The improved self-baking electrode of this invention has an outer electrically and thermally conductive casing, and an electrically and thermally conductive tubular shell positioned inside the casing to form an annular space between the shell and the casing. These elements will generally be round in shape, however, other geometric figures, such as ovals, hexagons, octagons and the like can be used with satisfactory results. The casing and the shell are connected and held in their relative positions to each other by electrically and thermally conductive support means, which can be, in general, any shape such as fins, studs, ribs and the like. The annular space between the casing and the shell is filled with a heat insulating material. Typical heat insulating materials include the cements used in furnace mortar and brick work which will be described more in detail hereinafter. The space inside the shell is fiiled with an electrically and thermally conductive, thermally sensitive, carbon-forming composition or paste. Typical examples include the electrode past-es normally employed in self baking types of electrodes. which also will be described in more detail hereinafter.

The casings which can be constructed from non-metallic materials of construction such as electrically conductive resins, structural carbon and graphite, fused silica and the like or metallic materials which are either ferrous materials such as carbon steel, cast iron, the various stainless steels, copper-coated steel, alloys of iron and aluminum and the like as well as the non-ferrous materials such as nickel, various nickel alloys of nickel and tin, alloys of nickel and copper, titanium, and the like. In most instances, metallic materials will be preferred. In most processes, ferrous materials will be satisfactory since they are relatively inexpensive and are electrically and thermally conductive. Of the ferrous materials, canbon steel is especially preferred. In some instances, however, when the temperatures inside the furnaces are relatively high, such as in the production of elemental phosphorus, it is preferred to construct the casing primarily from carbon steel and use a relatively thin layer of a highly electrically conductive material such as copper, aluminum and various highly electrically conductive alloys. This preferred embodiment enables a reduction in the operating temperature of the casing by reducing the electrical resistance and retains a relatively low cost casing. In most instances copper will be the preferred material for plating the carbon steel. The thickness of the layer of the highly conductive material will be dependent upon the particular electrode and the thickness of the casing. In most in stances the layer will be from about 1.0% to about 25% of the total thickness of the casing.

The shell and support means can generally be constructed of carbon steel, however, any of the materials which are suitable for the casing can be used for either the shell or the support means.

The heat insulating material can be, in general, any material which is thermally and electrically stable up to about 700 C. and has a thermal conductivity coeflicient of below about 12 B.t.u./hr./sq. ft./ F./ in. at about 500 F. In most instances it is preferred to use an insulating material which will undergo a hydraulic set at temperatures as low as about 25 C. and will undergo a thermal set at temperatures of about 600 C. Suitable materials include those which have thermal and electrical stability equivalent to the various refractory materials such as a high alumina content cement, cements prepared from bauxite clay, chrome brick, fireclay brick and the like. Especially preferred are the high alumina content cements that have a thermal conductivity coefficient of about 2 to B.t.u./hr./sq. ft./ F./in. at about 500 F.

The paste which is thermally and electrically conductive and which is partially decomposed to form solid carbon is typically a blend of carbon granules and a high temperature pitch such as that obtained from petroleum and coal tar distillation. The carbon granules are generally smaller than about of an inch and generally constitute from about 75% to about 83% by weight of the mixture, with the pitch constituting the remainder of the material. Suitable materials include those known in the art used in the traditional self-baking electrodes.

Electrode breakage is significantly reduced in the electrode of this invention because weakspots are not developed in the carbon and mechanical linkages are not employed in the carbon portion of the electrode. The invention therefore enables the use of larger electrodes with appreciable less electrode consumption. The electrodes of this invention can also be continuous electrodes, that is, columns can be built containing the casing, support means and shell upon the partially consumed electrode and then the cement and paste pours into place.

Electrodes for electric furnaces can be either solid or hollow. The solid electrode is utilized on furnaces where the furnace burden is fed to the furnace externally to the electrode. Hollow electrodes have one or more passages running the length of the electrode and thereby enable the burden to be fed through the electrode to the furnace hearth. Hollow electrodes are also used to provide a passage for removal of the gases which are produced in the furnace. The electrode of this invention can be either the solid type or the hollow type as desired for the particular use. In the hollow electrode of this invention at least one tubular member runs inside the paste the length of the electrode. This member can be constructed from any of the materials used for the casing, however, in most instances it will be made of carbon steel since the temperatures inside the passage will normally not be as high as those outside the electrode, since the tubular member will not generally carry the electric current. In most instances the tubular member will be round in shape, however, other shapes can be used if desired. In most instances only one tubular member is necessary, however, if desired a plurality of members can be used.

Two specific embodiments of the invention will now be described with reference to the accompanying drawings 4 in which FIGURES l, 2 and 3 refer to a solid electrode and FIGURES 4, 5 and 6 refer to a hollow electrode.

FIGURE 1 is a longitudinal sectional view of a phosphorous furnace equipped with a solid electrode of this invention.

FIGURE 2 is an enlarged view of a horizontal segment taken along line 2-2. in the electrode of FIGURE 1.

FIGURE 3 is an enlarged longitudinal sectional view of a segment of the lower portion of the electrode of FIGURE 1.

FIGURE 4 is a longitudinal sectional view of an electric furnace equipped with a hollow electrode of this invention.

FIGURE 5 is an enlarged view of horizontal segment taken along line 4-4 in the electrode of FIGURE 4.

FIGURE 6 is an enlarged longitudinal sectional view of a segment of the lower portion of the electrode of FIGURE 4.

With particular reference to FIGURE 1, a phosphorous furnace 10 is provided with a solid electrode 11. During operation, a burden is supplied to the furnace through a conventional means 12 and an electric current is supplied to the electrode 11 through a conduit 13. The carbon steel casing 14, receives the curent and transmits it through the carbon steel support means 15 (shown in FIGURE 2 which are fins extending the length of the electrode) and the carbon steel tubular shell 16 to an electrically and thermally conductive, thermally decomposable paste 17. During operation the temperature of the lower portion of the electrode 11 is about 2000 C. This results in the electrode being gradually consumed. The temperature outside the furnace housing above the sealing means 18 will generally approach the temperature of the air, however, due to the conduction of heat from inside the furnace, the paste 17 will be at about 200 C. At this temperature the paste is in a molten state but the hydrocarbons contained therein have not reached their decomposition temperature. The electrode paste 17 is a typical paste used in a self-baking electrode and contains carbon granules of a relatively small size, that is, smaller than about /3 diameter and contains about 17% by weight of a high temperature pitch derived from the distillation of coal tar. As the electrode Ill is consumed and moves down into the furnace, the temperature of the paste increases so that inside the furnace the temperature of the electrode paste ranges from about 400 C. at the upper portion just inside the furnace 10 to about 2000 C. at the lower portion. The hydrocarbons in the paste 17 decomposes to form hydrogen and carbon at temperatures of from about 400 C. to 600 C. The hydrogen escapes downwardly through the electrode tip since the tubular member 16 is protected from the heat and hot gases by a refractory cement 19 and therefore is not corroded. The carbon deposits in the voids between the carbon granules.

The casing 14, the support means 15 and the tubular member 16 are each constructed of carbon steel. The heat insulating material 19 is a high alumina content refractory cement having a thermal conductivity coefidcient of about 3 and undergoes a hydraulic set outside the furnace housing and a ceramic set inside the furnace when the temperature of the cement reaches about 700 C.

Referring to FIGURE 3, the casing 14 is the first of the members to be consumed since it is subjected to the hot gases inside the furnace throughout the period of time which requires for the electrode to be completely consumed and carries a large portion of the electric current. The heat insulating material 19 is later consumed because it is more stable to the higher temperatures than is carbon steel and does not carry the electric current. The carbon steel tubular shell 16 is consumed after it is directly exposed to the high temperatures by the consumption of the heat insulating material 19. The paste 1''] which has partially thermally decomposed forms a carbon which is extremely resistant to the high temperatures and to the corrosive gases contained within the furnace 10. An increased electrode life over any electrode heretofore known is achieved. For example, an electrode having an outside diameter of about 70 inches and operating at a current density of from about 2 to 4 amperes per square centimeter is consumed at the rate of about /3 inch per hour in a phosphorus furnace in which the temperature of the electrode tip is about 2000 C.

In particular reference to FIGURE 4 an electric furnace 20 is provided with a hollow electrode 21. A carbon steel inner tubular member 22 (shown in FIGURE provides a passage 23 throughout the length of the electrode 21 for the introduction of a burden to the furnace. During operation an electric current is supplied to the electrode 21 through conduit 24. The other elements of the electrode 21, that is the casing 25, the .support means 26 (shown in FIGURE 5), the tubular shell 27, the refractory cement 28 and the paste 29 are each constructed from the same materials and each function in the manner as described in reference to FIGURES 1, 2 and 3.

The electrode is sealed in the furnace by a conventional sealing means 30. The burden supplied to the furnace is about 150 F., thus the temperature of the carbon steel inner tubular member 22 thereby is below about 1200 C. for the major portion of the electrode 21. At the temperature no appreciable corrosion of the carbon steel inner tubular member occurs.

The size of the electrode, the thickness of the casing, shell and tubular member, if a hollow electrode is desired will be dependent upon the particular furnace design and can be determined from engineering guidelines established for electric furnaces. Additionally, the width of the space between the casing and the shell will be dependent upon the temperature outside the electrode, the heat insulating material used and the materials of construction used for the shell. These factors, of course, will be dependent upon the particular furnace and electrode design and the process in which the electric furnace is being used. For example, in the production of elemental phosphorus, satisfactory operation is achieved using an electrode having a tubular carbon steel casing, having a diameter of about 6 feet and a thickness of about 0.2 to about 0.3 inch, and a space of from about 2.2 to about 3.0 inches between the casing and a carbon steel shell (having a thickness of from about 0.08 inch to about 0.12 inch), which space is filled with a high alumina content refractory cement having a thermal conductivity coeificient of about 2 B.t.u./hr./sq. ft./ F./in. at about 500 F.

What is claimed is:

1. An electrode for an electric furnace comprising (a) an electrically conductive casing, (b) an electrically and thermally conductive tubular shell positioned inside said casing to form an annular space between said shell and said casing, (c) electrically and thermally conductive support means connecting said casing and said shell, (d) a heat insulating material filling said space between said shell and said casing and (e) an electrically and thermally conductive, thermally sensitive, carbon-forming paste inside said tubular shell.

2. An electrode according to claim 1 where said casing, said shell and said support means are each metallic materials.

3. An electrode according to claim 2 where said metallic materials are ferrous metals.

4. An electrode according to claim 1 wherein said support means are fins extending radially from said casing to said shell and extend the length of said electrode.

5. An electrode according to claim 4 wherein said casing, said shell and said support means are each constructed from a ferrous metal.

6. An electrode according to claim 5 wherein said shell and said support means are each carbon steel.

7. An electrode according to claim 6 wherein said casing is carbon steel.

8. An electrode according to claim '7 wherein said casing contains a relatively thin layer of copper.

9. An electrode according to claim 2 wherein at least one inner tubular member extends the length of said paste to thereby provide a passage extending the length of said electrode.

10. An electrode according to claim 9 wherein said casing, said shell, said support means and said inner tubular member are each constructed from carbon steel.

References Cited UNITED STATES PATENTS 1,640,735 8/1927 Soderberg 1318 1,691,505 11/1928 Walther 13-18 2,764,539 9/ 1956 Horvitz 204-67 FOREIGN PATENTS 529,118 6/1931 Germany.

BERNARD A. GILHEANY, Primary Examiner. H. B. GILSON, Assistant Examiner. 

